Effects of two sliding motions on the superlubricity and wear of self-mated bearing steel lubricated by aqueous glycerol with and without nanodiamonds

Comparative experiments were performed to demonstrate how the lubricated sliding friction and wear behavior of a bearing steel can be affected by the type of relative motion. Data from rotary motion (ball-on-disk tests) were compared to that for reciprocating motion (high frequency ball-on-flat tests) using the same contact pressure, average velocity, temperature and humidity. Two lubricants were compared. The first was aqueous glycerol and the second was aqueous glycerol containing 0.1% w nanodiamonds. The aqueous glycerol solution showed superlubricity effects in rotary testing because its friction coefficient was lower than 0.01. This effect was attributed to a stable protective matrix comprised of free and bound water layers in aqueous glycerol and a thick film lubrication condition. Under reciprocating conditions, it is proposed that the protective matrix was damaged due to the repetitive application of compressive stress and boundary lubrication condition. As a result, there was no superlubricity behavior observed in reciprocating tests. In the rotary test method, the nanodiamond additives (dispersed in glycerol) and wear debris accumulate at the contact zone. That mixture grows into a large abrasive particle mix. However, during reciprocation there are fewer nanodiamonds and wear debris particles in the contact zone. They polish and smooth the surface, as indicated by the post wear surface images. The abrasive nature of nanodiamonds increases the micro polishing effect and thus decreases friction, significantly. It is shown that the lubricity behavior of a lubricant and additive can be influenced by the direction of motion used in the test method.